Interference Fringe Intensity Research Articles

Robust Phase Reconstruction Using Random Three Step Phase-Shift in Wavelength Tuning Interferometer

The standard stochastic two-step phase shift phase demodulation method analyzes the tangent function of the phase to be measured to extract the phase to be evaluated. It is necessary to eliminate the background light intensity of interference fringes, which has a significant impact on phase reconstruction accuracy. A random three-step phase shift algorithm (RTP) is suggested in this study. The phase shift between two adjacent interference fringes is extracted using the least squares approach in conjunction with a K-order twodimensional polynomial. Additionally, it employs the interference fringe graph’s difference operation to reduce the background light intensity and actualize the restoration of the measurement phase. The experimental and simulations results reveal that phase shift, luminous non-uniformity, and random noise have little effect on the random three-step phase shift method and have strong environmental adaptability.

Design of an All-Fiber Fabry-Perot Sensor for Strain Measurement in Radiative Environment

A length-matched micro Fabry-Perot (FP) interferometer is proposed for strain measurement under irradiation environment. Theoretical simulation shows that a well length-matched FP sensor can achieve a very low drift of the cavity length and strain sensitivity in irradiation environment. In experiment, such an FP cavity is realized by laser micromachining. It shows a low cavity length drift of −0.037 µm and a strain sensitivity deviation of 0.52%, respectively, under gamma irradiation. Meanwhile, the intensity of interference fringes is also stable. As a result, such a length-matched FP cavity is a very promising candidate for strain sensing in radiative environments.

Increased intensity of interference fringes in Digital Holography

Digital holographic interferometry is a full field optical technique, used in the measurement of dynamic and non-contact events, this technique works with arrays where the initial acquisition of interference fringes is not necessary, but rather involves the superposition of two wave fronts. Where from the holograms and by means of a Fourier window-based processing it is feasible to recover the information of both intensity and phase. However, the interference fringes resulting from the demodulation of the holograms may have low intensity, affecting their subsequent analysis. In this work, the combination of the holographic techniques, filters and the superposition principle is proposed to increase the intensity of the fringes, the results show that using the Fourier method in combination with the superposition theorem is possible to obtain greater intensity between the minimum and maximum of the fringes.

Quantitative determination of elastic and inelastic attenuation coefficients by off-axis electron holography

Off-axis electron holography is a well-established transmission electron microscopy technique, typically employed to investigate electric and magnetic fields in and around nanoscale materials, which modify the phase of the reconstructed electron wave function. Here, we elaborate on a detailed analysis of the two characteristic intensity terms that are completing the electron hologram, the conventional image intensity and the interference fringe intensity. We show how both are related to elastic and inelastic scattering absorption at the sample and how they may be separated to analyze the chemical composition of the sample. Since scattering absorption is aperture dependent, a quantitative determination of the corresponding attenuation coefficients (reciprocal mean free path lengths) requires the use of holographic image modi with well-defined objective aperture stops in the back-focal plane of the objective lens. The proposed method extends quantitative electron holography to a correlated three-in-one characterization of electric and magnetic fields, Z-contrast and dielectric losses in materials.

Channeled spectropolarimetry based on division of aperture and field of view

In order to obtain accurate image, spectrum and polarization state of target by the interferometric channeled spectropolarimeter, the interferogram and the image need to be separated. Although it can be achieved by digital image processing technology, heavy computations with approximation would be introduced. In the application of channeled spectropolarimetry, an inevitable crosstalk will be present between channels of the interferogram formed on the CCD. Spatial filtering in the optical path difference domain will generate a loss of spectral resolution and the distortion of the recovered spectrum. To overcome these drawbacks, a static imaging channeled spectropolarimeter based on division of aperture and field of view is presented. The aperture is divided by a polarization array, which consists of two polarizers with their transmission axes perpendicular to each other. The field of view is divided by a pair of lenses with the same focal lengths. The spectral modulation module is composed of an achromatic quarter wave plate, a retarder and a polarization array. The interference system consists of an achromatic half wave plate, a Wollaston prism, and a Savart polariscope. Two pairs of in-phase and anti-phase interferogram with different intensity modulations can be obtained simultaneously on a single detector array. The pure image of the target is acquired by the summation of the four interferograms. The background intensity is removed by the subtraction of the interferograms with in-phase and anti-phase, and the pure interference fringes can be acquired. By the summation and subtraction of the two pure interference fringes, the single channeled interference fringes corresponding to spectrum of intensity and linear polarization state can be obtained. Spectral and polarization information of the target are acquired by Fourier transform of the single channeled interference fringes. Compared with previous instruments, the described model has the significant advantage that the background intensity can be removed from the hardware of the layout, and thus avoiding the spatial filtering in the optical path difference domain. The obtained spectra have the same resolutions as those obtained from the interference system, and the distortion of the recovered spectrum can also be vanished. Since there is neither rotating part nor moving part, the system is relatively robust. In the present paper, the principle of the instrument is described, and the interference fringe intensity distribution formula is obtained and analyzed. The performance of the system is demonstrated through a numerical simulation. This work will provide an important theoretical basis and the practical instruction for designing a new type of imaging sepctropolarimeter and its engineering applications.

Technology of polarization interference imaging spectral based on pupil division and angle shear

With the combination of pupil division, Wollaston prism angle shear and Savart plate lateral shear interference, a new technology that can aquire the message of imaging, polarization and spectrum simultaneously is presented. The system works at the pushing mode with neither mechanical movable nor electrically tunable device, and four quarters of the single charge-coupled device can get four interferograms of different polarization states (two polarization-difference spectral imagings). The device is discussed in detail, and the relationship between azimuth of the prism principal cross section and the output light intensity is analyzed, and then system parameters are optimized. In the paper, we derive the interference fringe intensity distribution formula, use computer numerical simulation to verify the feasibility and accuracy of this method, and thus provide an important theory basis and the practice instruction for designing a new type of imaging spectrometer and its engineering applications.

Use of Fiber Interferometer for AFM Cantilever Probe Displacement Control

This investigation presents a fibre-optic Fabry-Perot interferometer as a displacement sensor in an atomic force microsope (AFM). A simple model of light wave transmission between two fibres with the same core diameter is proposed to determine the theoretical equation of light intensity of interference fringes from the fibre-optic Fabry-Perot interferometer. By replacing an AFM cantilever with a movable reflective mirror, the variations of relative light intensity of the interference fringes with the spacing between the fibre and the mirror were recorded. The theoretical equation for the light intensity of interference fringes was close to those obtained experimentally. Finally, a fibre-optic Fabry-Perot interferometer was operated in an AFM to image a two-dimensional phase array with a pitch of 4 µm and a depth of 150 nm.

Optical biopsy: noninvasive tissue evaluation utilizing optical coherence tomography

The accurate noninvasive assessment of traumatic wound depth and serial evaluation of wound healing is an active and ongoing research goal. A large number of noninvasive modalities have been evaluated including the use of vital dyes, indocyanine green dye fluorescence, fluorescein fluorometry, laser Doppler flowmetry (LDF), thermography, ultrasound, nuclear magnetic resonance imaging, and spectral analysis of light reflectance. Due to various limiting factors, none of these technologies has greatly improved the clinician's ability to assess tissue viability beyond simple observation. Biopsy and histopathology remain the gold standard for tissue evaluation. Histologic assessment is invasive and has a slow turn‐around time, even when frozen section techniques are utilized. Polarization‐sensitive optical coherence tomography (PS‐OCT) and optical coherence tomography/optical Doppler tomography (OCT/ODT) devices have recently been developed that have the potential to assess traumatic injury noninvasively at the bedside in a real‐time mode. The technology is analogous to ultrasound except that the images are created with echoes of light rather than sound. Polarization‐sensitive optical coherence tomography uses coherence gating to image tissue birefringence with a high degree of spatial resolution. Skin contains collagen, a weakly birefringent material. When collagen is subjected to temperatures between 56 and 65°C, the collagen denatures and loses its birefringence. When the polarization state of light reflects from various depths in a tissue, PS‐OCT can determine depth based on condition of the collagen. The OCT/ODT technology has the ability to noninvasively image in vivo blood flow with high spatial resolution. Optical Doppler tomography essentially combines LDF with OCT to produce high‐resolution tomographic images of static and moving constituents in biologic tissues. By using a Michelson interferometer with a low coherence light source, ODT can measure amplitude and frequency of the interference fringe intensity generated between the reference and target arm to form a combined structural and velocity image. Using skin in a rodent model, we compared the results of this instrument to standard histopathology and frozen section techniques. We compared four different histology staining techniques, H&E, Masson's trichrome, Movat's pentachrome, and nitroblue tetrazolium (a vital stain), to the PS‐OCT and OCT/ODT device results. The OCT measurement contained less variability site‐to‐site when compared with standard histology techniques. We also found the PS‐OCT technique provided an accurate assessment of wound depth when compared with 366 histology specimens (r2 = 0.86). Further studies are planned with OCT/ODT technology in skin and other tissues. Preliminary work with this device in the airway, gastrointestinal tract, and muscle appears promising. Utilization of these combined noninvasive imaging technologies has the potential to improve assessment of traumatically‐ and thermally‐injured patients in real time, thereby improving overall wound care. Funding: United States Department of Defense.

Performance Evaluation of Fabry-Perot Temperature Sensors in Nuclear Power Plant Measurements

The Fiso Fabry-Perot fiber-optic temperature sensor was selected for performance evaluation and for potential application in nuclear power plants because of its unique interferometric sensing mechanism and data-processing technique, and its commercial availability. It employs a Fizeau interferometer and a charge-coupled device array to locate the position of the maximum interference fringe intensity, which is directly related to the environmental temperature. Consequently, the basic sensing mechanism is independent of the absolute transmitted light intensity, which is the most likely parameter to be affected by external harsh environments such as nuclear irradiation, high pressure/temperature, and cyclical vibration.This paper reports research on the performance of two Fiso Fabry-Perot temperature sensors in environmental conditions expected in nuclear power plants during both normal and abnormal (i.e., accident) conditions. The environmental conditions simulated in this paper include gamma-only (60Co) irradiation, pressure/temperature environmental transient, and mixed neutron/gamma field, respectively.The first sensor exhibited no failure or degradation in performance during and following gamma-only irradiation in which a total dose of 15 kGy was delivered at a dose rate of 2.5 kGy/h. Following gamma irradiation, this sensor was then tested for 10.75 days in a thermohydraulic environment prescribed by the Institute of Electrical and Electronics Engineers IEEE323-1983. Intermittent behavior was observed throughout the latter portions of this test, and degradation in performance occurred after the test. Visual evaluation after opening the sensor head indicated that the internal welding methodology was the primary contributor to the observed behavior during this test. Further consultation with the vendor shows that the robustness and reliability of Fiso sensors can be substantially improved by modifying the internal welding methods.The second Fiso temperature sensor was tested in a mixed neutron/gamma environment, in which the total neutron fluence was 2.6 × 1016 neutrons/cm2 and the total gamma dose was 1.09 × 108 rads. During the initial calibration following completion of the test, the sensor exhibited a fixed offset of 18.9°C (34°F offset) but responded linearly to change in temperature. An annealing phenomenon was observed as the temperature increased, which reduced the offset by ~63%. This observed phenomenon will be very valuable in the model building for on-line maintenance and calibration of the Fiso temperature sensor in a mixed neutron/gamma irradiation field. Fiso Technologies in Canada has suggested that the temperature offset may also be reduced by improved welding methods. If these shifts can be compensated in the on-line sensing or calibration model of the Fiso sensor, then this type of fiber optic sensor will be competitive in nuclear power plant applications.

System model identification of a Fabry-Perot fiber optic sensing system

A Fabry-Perot fiber optic temperature sensing system is presented in this paper. It uses a Fabry-Perot interferometer (FPI) to transform the environmental temperature into modulated reflected light. This light is directed to a patented light cross-correlator that locates the position of the maximum interference fringe intensity, which is detected by a linear CCD array. Therefore, the actual observed data is the position of the CCD pixel detecting the maximum interference fringe intensity rather than the light intensity itself. Consequently, this sensing mechanism is tolerant to the loss of light power that may result from external effects. Based on an analysis of the entire sensing system, a theoretical dynamic model was developed, which shows that the system dynamic response depends on the heat transfer process in the sensor head and the signal processing in the signal conditioner. An experimental method was developed to validate the theoretical model. Two empirical dynamic models are also obtained from the experimental data. Comparing the theoretical model with the empirical models, the poles result from the heat transfer process in the sensor head.

Electrostriction measurements on low permittivity dielectric materials

To measure the electrostrictive effects in low permittivity materials, extremely sensitive instrumentation is required. A modified compressometer for resolving fractional changes in capacitance on the order of 10 −6 is used in this work, along with a modified single beam interferometer capable of sub-angstrom resolution in displacement. For the compressometric method, a high sensitivity capacitance bridge, GenRad 1615, is coupled with two lock-in amplifiers to detect attofarad (10 −18F) level capacitance changes caused by in-phase cyclic uniaxial stresses on samples. The interferometer is a Michelson –Morley type instrument modified to detect changes in interference fringe intensity for very small changes in path length. The measurements confirmed by both techniques are used to establish a set of reliable and accurate data of electrostriction coefficients for low permittivity materials. Using these recent data, along with widely accepted data on ferroelectric materials and soft polymers, the linear relationship between electrostriction coefficient (Q) and the ratio of elastic compliance over dielectric permittivity ( s/ε 0ε r) is obtained. This leads to an effective way to predict the electrostriction coefficient in dielectric materials.

Methods for producing two-coordinate holographic diffraction gratings

Methods for the preparation of two-coordinate holographic diffraction gratings using a two-, three- and four-beam interferometer are considered. The analytical relationship of the interference fringe intensity distribution in the output aperture of these interferometers is presented. Experimental results are given.

Quantitative Analysis of the Intensity of Interference Fringes of X-Ray Diffraction Line Profiles

X-Ray diffraction spectra of thin films may show characteristic interference fringe patterns which wash out with increasing surface roughness. From the intensity of the secondary maxima quantitative data on the roughness can be derived. Model calculations are presented assuming that the distribution of heights about the mean surface level is Gaussian. The theory is applied to explain the diffraction spectra of single-crystal silver films. It is shown that some uncertainty in the results remains due to the influence of lattice distortions

Measurement of deposition rate in matrix spectroscopy with a small laser

A set-up is reported for measurement of the deposition rate of transparent matrix materials typically used in matrix isolation spectroscopy. Its main features consist in measurement of intensity of interference fringes produced by the growing matrix layer on the light of a small laser by a simple power meter. A typical example for an Ar matrix is shown.

Proposed Interferometric Method of Measurement of Roughness and Autocorrelation Function in Smooth-Finished Surfaces

The proposed interferometric method allows measurement of rms roughness and autocorrelation function in smooth-finished surfaces. In a Jamin interferometer, pairs of coherent beams with variable magnitudes of small lateral shear are produced that are reflected from the test surface and intefere. The intensity of interference fringes is recorded in dependence on the shear of the beams. The differences between maximal and minimal intensities (or visibility) decrease in dependence on the shear, and the course of the decrease corresponds to that of the autocorrelation function. At a fairly great shear the visibility stabilizes at a certain value from which the surface roughness is derived. The theoretical basis of the method is presented, the accuracy is discussed, and the measuring device is proposed.